Comparison of WAG and Continuous Enriched-Gas Injection as Miscible Processes in Sadlerochit Core

Abstract

ABSTRACT Short core displacements were conducted to evaluate miscible condensing-gas drive for the Sadlerochit reservoir of the Prudhoe Bay field. The cores were from well DS 14-5. The enriched gas was a 60-40 blend on a mole basis of rich Sadlerochit gas and the liquids from the Field Fuel Gas Unit (FFGU) and the separators at Flow Station 3. The experimental program consisted of four floods conducted in each of four cores for a total of sixteen displacements. Each set of floods consisted of: a gravity-stabilized continuous gas injection displacement with the core at connate water saturation;a gravity-stabilized continuous gas injection displacement with the core at waterflood residual oil saturation;a simultaneous injection brine enrichedgas flood at a 4 to 1 brine-to-gas ratio (as determined at reservoir conditions) with the core at connate water saturation;a simultaneous injection brine enrichedgas flood at a 4 to 1 brine-to-gas ratio with the core at waterflood residual oil saturation. All displacements yielded approximately the same residual oil saturation averaging 2.26% STPV (i.e., stock tank pore volume). The range of values was 0.88 to 3.74% STPV. The gravity stabilized displacements were piston-like, consistent with the experimental design. The piston-like displacement coupled with the high recoveries indicated rock pore structure did not adversely affect the displacement. In the simultaneous injection displacements with brine, the oil production was dispersed due to the high fractional flow of brine. However, mobile brine did not affect the ultimate recovery. Trapping or bypassing of oil was not observed. Trapping was observed in a simultaneous injection displacement conducted in strongly water-wet Berea using the same fluids at a 4 to 1 brine-to-enriched-gas ratio. For this flood, the residual oil saturation was 19.4% STPV. Oil production curves for the simultaneous injection displacements before and after waterflooding were identical except for the early production of the waterflood oil saturation in the non-waterflooded cores. Thus for this linear system, the timing of oil recovery is better for the WAG (i.e., water-alternating-gas ) process operated in the secondary mode as opposed to the tertiary mode.